Effects of Various pH Conditions on Authigenic Chlorite and Kaolinite Surface Characteristics Using SEM and X-Ray Microanalysis

A. Umran Dogan, S. W. K. Leung

Authigenic chlorite and kaolinite are major diagenetic minerals in petroleum-bearing sandstone units. Authigenic chlorite in marine sandstone units of the Upper Cretaceous Parkman formation of Wyoming occurs as grain coatings which line pores. Chlorite plates developed on grain surfaces perpendicular to grain margins, with individual crystals being about 3 to 5 microns in diameter and 0.1 micron thick. Authigenic kaolinite occurs as stacked pseudohexagonal booklets that fill pores. Its crystal diameter is about 10 microns.

Well-developed chlorite and kaolinite crystals coexist in many sandstone units. Some workers claim that chlorite is extremely sensitive to acid, and some claim that acidic conditions are required to form kaolinite, although fresh water need not be the only source of acidity. In the Parkman formation, chlorite is well preserved in pores. Therefore, it appears that the chlorite-bearing units may not have been subjected to acidic conditions after chlorite was formed. However, kaolinite in these units appears to have precipitated after chlorite formation. If this is so, then acidic conditions required to form kaolinite would have destroyed earlier formed chlorite.

To test this problem, a laboratory experiment was designed to show effects of varying pH conditions on these crystals. Experiments were done under atmospheric conditions. Solutions were maintained with pH of 4, 6, 7, 8, and 10. Physical chemical changes on crystal surfaces were studied using scanning electron microscopy, x-ray microanalysis, and Shape Analyzer. Details of the precipitation mechanism and kinetics of equilibrium of these minerals will be discussed.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.